Is it worth focusing on food?

If you're going tall, definitely, food is important. After production, my building is focused on growth. 

If you're going wide, not so much, but you still need a high level fortress to produce the best units, and high level conclaves for research and mana, so yes, it's still worth the time, and effort, imo.

also don't forget that once your city etc gets bigger/ gains alevel, you can take the food enchantment off your city etc will not get smaller, although you may lose population, but that only matters when you want to grow the city larger again.

Generally, food isn't worth much. Personally, I have only reached one level 4 city once...

If you manage to spread quickly, those first few cities can reach lvl 3 or 4 and benefit from having at least 3 food. After that (let's say from your 5th city on, but that greatly depends on map and playstyle), 2 food is more than adequate. Even 1 might suffice, if it has 5/6 prdocution.

Strangely enough, the game does not penalize low population, so if you think you will never reach the next threshold before the end of your game, you may as well spend all the population of a city, through pioneers or the Sacrifice spell (if you have it).

like always it depends a lot on your playstyle and the map settings, food can be very important and Fallenchar is not correct with his general statement.

If you want to stretch your empire wide on a big map by settling new and conquering enemy cities, you need fortresses with lots of food to reach the lvl 4 prison and lvl 5 onyx throne improvements to counter the city count penalty. If you want to create a conclave city that utilizes its essence yield to the fullest and therefore becomes a research and mana powerhouse, you need lots of food. If you need to maintain a big army financially and if you want to counter low production locations in your empire, you need lots of food to reach the higher level town improvements.

So food is worth a lot even if you play on a big map and if you want to establish and maintain a vast empire.

As said above, Growth is more important than food, but higher food yield also means higher growth, so in that case i think food is also important. Depends on which type of city of course.

And don't forget that if a city levels up, not only the unique building that is useful, but their yield will increase too, example: a level 2 conclave have 10 research, when it reach level 3 it will become like 17 or 20, it's a passive bonus of level up, not because of any spells, any abilities, or any buildings or any research produce, the bonus is given for free to your conclave.

Corrected that for you. The first correction is meant as a joke - obviously, but feel free to view it otherwise  -.

Anyway, it would be interesting if someone with math-skills could calculate when it would be more beneficial to build another pioneer or to keep growing to the next level, under various conditions (nr of cities, type of city, map size, turns played, etc)

Because of the way you get more growth the bigger the difference between your current population and the amount of food you have currently, it makes sense to decide on one town that you you will use for pioneers and Sacrifice, because the population will be replaced faster. A level two town works quite well for this.

I've only once got a settlement to level five, because I've generally won the game before then, but this does depend on the size of the map, the way you play, etc. etc. In general I've found the biggest factor in determining maximum settlement level was actually all the global food bonuses from my towns.

I don't care a great deal about food production when I found a town, the only thing I'm really concerned about is how quickly I can get it to level two. Essence, production and getting important resources are much more important to me than food.

Focus on food as in building those improvements first? No. You only get 3 growth from food max and it will decline as population grows. No point in getting those improvements until you need them. (remember you use population for pioneers early on, so it will be very slow in the start) A high food tile is good for spamming pioneers, not becoming a big city quick. 

Focus on food as in reaching city levels as soon as possible, yes. Use consulates(outpost improvement), spells and Wildland heads to improve growth, when you get 6+ growth your cities should get there very quick. 

A high essence, high material city is the optimal for quickest lvl 5 city. Use the essence slots on food and growth enchants, get the food buildings up faster and you have the ability to get consulates up as well for growth, all that makes it far easier to get the precious lvl 5 on your city.

...and even if you're building high, not wide, you want lots of pioneers to control shards and resources.  

Ah, this was the key piece i was missing!

That makes growth a lot stronger than i thought, because while buildings can generally get me another +1/+2, growth can get me a much bigger bonuses.

I hadn't realised that yields go up when city size goes up. However even if that's correct, I don't think it makes that big a difference, because it takes so long to reach higher levels. If it's going to take you 80 turns to go up a level, in that time with high production you could have built ten buildings, and you would get the incremental benefit as each building is completed, rather than having to wait eighty turns to get a new level. My games are generally over before I get to a level five settlement (although I did get one there in my last game).

I tend to treat higher population levels (beyond 2) as a nice to have rather than a goal in itself. I tend to only build food structures when there is either nothing better to build or I'm approaching a population cap. I will build growth sometimes if I'm approaching a population level, assuming I don't need to build gold.

Perhaps you haven't played that many games, but yes it is true, the passive bonus exist, it's there since vanilla FE (FE v1.00), it's even there since Elemental War of Magic (if you still remember this game).

It doesn't take that long to level up the city to high level, things has changed since FE, in FE you need high food yield beside high growth to reach level 3 or even 4 fast, in LH you just need growth, food is not as important because the maximum population needed for level up has been lowered (the change log posted by Derek doesn't lie), and it's easier now to obtain food (especially for town type city).

That being said, i'd still advise not focusing in food because of that, food is easier to increase now than in FE, however growth is still the same problem, but like in FE there are several ways to solve this, the spell sovereign call being the easiest and fastest way.

A balanced focus with the least in food is useful for most situation, but in the end it depends on individual strategy and play style.

Also this is just for info, the passive bonus in LH is not as useful as in FE, because in LH when a city reach lv 2, the type of city now give much more bonus than in FE, example is fortress in FE only give +1 level to trained unit, now it give many bonus beside that. However the passive bonus (not town specialization bonus) is massive when a city reach level 3, even more so for level 4 and 5, from real experience i'll take conclave as example, at level 2 a conclave without any modifier for research produce 10 research, at level 5 it can reach 40 research, with modifier, the modifier will give higher bonus because the base number which it modify increase so much (a conclave with 100 research is not a myth). In FE, +x % research building like "sage" is not as useful as the flat version increase like "school", however in LH it works the opposite, the +x% being more useful now because city in LH level up much faster than in FE, which is why food is not as important in LH as in FE.

biggest problem is that level 5 is the end

there needs to be level 6 & 7 and a cap on how much other cities can affect a single city's growth... spamming cities mindlessly shouldn't be so optimal

My last Ridiculous game (medium map) lasted 234 turns, something like that anyway. That was the longest game I've played, and the only time I've got to a level five settlement. If I don't think I'm clearly winning after 100 turns, something has gone wrong. To put it another way, by the time I get a level four settlement, the game is practically over. So while bigger settlements are nice, they're not really something I focus on.

Indeed, mostly level 3 cities is enough. The problem of leveling city right now is growth, as i've said above food is not a problem in LH (it is in FE). And back to topic, focusing on food is not worth in LH, it can be good in FE, but not in LH.